1. Field of the Invention
The present invention relates to a zeolite sol which can be formed into a porous film that excels in dielectric properties, adhesion, film consistency and mechanical strength; a composition for film formation; a porous film and a method for forming the same; and a semiconductor device which contains the porous film inside.
2. Description of the Related Art
In the fabrication of semiconductor integrated circuits, as the circuits are packed tighter, an increase in interconnection capacitance, which is a parasitic capacitance between metal interconnections, leads to an increase in interconnection delay time, thereby hindering the enhancement of the performance of semiconductor circuits. The interconnection delay time is called an RC delay which is in proportion to the product of the electric resistance of the metal interconnections and the static capacitance between the interconnections. Reducing the interconnection delay time requires reducing the resistance of metal interconnections or the interconnection capacitance.
The reduction in resistance of the interconnection metal and the interconnection capacitance can prevent a densely packed semiconductor device from causing an interconnection delay, thereby realizing a finer and faster semiconductor device with reduced power consumption.
In an attempt to reduce the resistance of metal interconnections, in recent years, metallic copper interconnections have been employed more than conventional aluminum interconnections in the structure of a semiconductor device. However, use of this structure alone has limits in the enhancement of the performance, so the reduction in interconnection capacitance is an urgent necessity for higher performance of semiconductors.
One method for reducing interconnection capacitance is to reduce the relative permittivity (dielectric constant) of an interlevel insulator film disposed between metal interconnections. As such an insulator film with a low relative permittivity, it has been considered to use a porous film instead of a silicon oxide film which had been used conventionally. Since a porous film is the only practical material having a relative permittivity of 2.0 or less which is suitable for an interlevel insulator film, various methods for forming a porous film have been proposed.
A first method for forming a porous film is as follows: a precursor solution of a siloxane polymer containing a thermally unstable organic component is synthesized; then the precursor solution is applied on the substrate to form a coating film; and later, a heat treatment is applied to decompose and volatilize the organic component. The result is a number of micro-pores formed in the film.
As a second method for forming a porous film, it is well known to carry out processing as follows: a silica sol solution is applied onto a substrate by coating or using a CVD method so as to form a wet gel; and then the silica sol is subjected to a condensation reaction while restricting volume reduction by controlling the speed of the evaporation of the solvent from the wet gel.
As a third method for forming a porous film, it is well known that a silica micro-particle solution is applied on a substrate to form a coating film, and then the coating film is sintered to form a number of micro-pores between silica micro-particles.
As a fourth method, Japanese Patent Provisional Publication No. 2000-44875 proposes a composition for porous film formation which contains (A) a component expressed by R1′mSi(OR2′)4-m (in the formula, R1′ and R2′ can be the same or different from each other and each indicates a univalent organic radical, and m is an integer of 0 to 2); (B) a metal chelate compound; and (C) a compound having a polyalkylene oxide structure.
However, these methods have respective major drawbacks as follows.
In the first method for forming a porous film, the synthesis of the precursor solution of the siloxane polymer increases the cost. In addition, the formation of the coating film by coating the precursor solution increases the amount of silanol groups remaining in the coating film, which causes degassing phenomenon indicating the evaporation of water and the like in the heat treatment process that is conducted later and which also deteriorates the film quality due to the porous film absorbing humidity.
In the second method for forming a porous film, the speed control of the evaporation of the solvent from the wet gel requires a special type of coating device, which increases the cost. In addition, a significant amount of silanol remains on the surface of the micro-pores which must be silanized because otherwise hygroscopicity is high and the film quality decreases. The silanization makes the process more complicated. In the case where a wet gel is formed by the CVD process, it is necessary to use a special type of CVD device which is different from the plasma CVD device generally used in the semiconductor process, thereby also increasing the cost.
In the third method for forming a porous film, the diameter of the micro-pores formed between the silica micro-particles, which is determined by the accumulation structure of the silica micro-particles that are accumulated geometrically, becomes very large. This makes it difficult to set the relative permittivity of the porous film to 2 or below.
In the case of the fourth method, out of the three components (A), (B), and (C), the metal chelate compound of (B) is essential to increase the compatibility of the components (A) and (C), and to make the thickness of the coating film uniform after being hardened. However, it is not preferable because it makes the manufacturing process complicated and increases the cost. Therefore, it is desired to develop a material which enables a homogeneous solution to be formed without a chelate component and the coating film to be flat after being hardened.
In comparison to the method for forming a porous member which has conventionally been used, it has been found that a porous member having a channel structure of mesopore size (micro-pores with diameters of 2 nm to 50 nm) can be formed in the following method: alumino silicate, silica or the like is condensed while using a micelle made from a surface active agent as a mold so as to form the structure, and then the surface active agent component is removed by sintering or solvent extraction. For example, Inagaki et al. proposes a method for the reaction of polysilicates in water while using a surface active agent as a mold (J. Chem. Soc. Chem. Commun., p. 680, 1993). Furthermore, Japanese Patent Provisional Publication No. 9-194298 discloses that tetraalkoxysilane is reacted in an acidic condition in water while using a surface active agent as a mold, and is applied onto the substrate so as to form a silica porous film having micro-pores of diameters of 1 to 2 nm.
However, these methods have problems as follows. In the former method, a powdered porous member can be easily formed, but it is impossible to form a porous film as a thin film on a substrate which is used for the fabrication of semiconductor devices. In the latter method, a porous member can be formed into a thin film, but it is impossible to control the orientation of micro-pores, and it is also impossible to form a uniform thin film in a wide area.
Japanese Patent Provisional Publication No. 2001-130911 discloses a method for forming a silica mesoporous thin film by using a mixture of an acid hydrolysis condensate of a silicon alkoxide and a surface active agent after adjusting the mixture to pH3 or below for stabilization.
However, in this method, too, the restriction of the solute concentration makes it difficult to properly control the thickness of a coating film, thereby making it difficult to apply it to a practical semiconductor fabrication process. When this solution is diluted with water, the thickness of the coating film becomes controllable, but the speed of polycondensation of the silica component increases to lose stability of the coating solution.
On the other hand, use of zeolite as the silicon oxide film has been attempted. For example, in Adv. Mater., 2001, 13, No. 10, May 17, p. 746, a zeolite film is prepared by coating tetrapropylammonium hydroxide, tetraethoxysilane and water onto a silicon wafer and processing them in a sealed container. Some other methods have been proposed. For example, according to AIChJ., 42, 3020, (1996), a seed crystal is coated on a supporting member in an autoclave, and a thin water gel solution is used to make only crystal growth progress. According to J. Chem. Soc. Chem. Comm., 1990, 755, a dry gel is turned into zeolite by a mixed vapor of ethylenediamine, triethylamine and water.
However, these methods form a zeolite film in a sealed container such as an autoclave, and are not practically used in a fabrication process for semiconductor devices.
On the other hand, Japanese Patent Publications Nos. 2001-115029 and 2001-203197 show that a coating solution excellent in dielectric properties is prepared by the hydrolysis and condensation of a silane compound. Because it is considered that a coefficient of elasticity must be 5 GPa or higher for use in the actual semiconductor fabrication process, these inventions cannot be said to be satisfactory for mechanical strength.
As described above, the conventional porous films have problems wherein the film consistency is low and the thickness is not properly controllable, and as a result, it is difficult even to prepare a coating film from the material using zeolite sol. Therefore, when a porous film using the conventional zeolite sol is integrated into multilayered interconnections of a semiconductor device as an insulator film, the porous film is difficult to be thinned, the film consistency is low, and the film thickness is not properly controllable. These problems decrease the yield of the semiconductor device fabrication or make the fabrication itself difficult. Furthermore, a porous film with a low mechanical strength deteriorates the reliability of the semiconductor device, even if it can be used for semiconductor fabrication.